From the prior art multi-stage change-speed transmissions with two countershafts and a central main shaft are known. In these, the individual gears are produced by activating in each case a gear ratio step that comprises a loose wheel, which is arranged on the main shaft, and a respective fixed wheel, which is arranged on each countershaft, which are in active engagement with one another. The activation of the gear ratio steps takes place by actuating a shift element by means of which the loose wheel concerned can be connected in a rotationally fixed manner to the main shaft. Accordingly, the loose wheels arranged on the main shaft rotate in the circumferential direction loosely along with the main shaft and are only connected in a rotationally fixed manner to the main shaft in order to transmit torque when the associated gear step is in use.
Chattering and rattling noises are produced by transmission components when not under load, such as loose wheels, synchronization rings and shifting sleeves, due to low-frequency torsional oscillations introduced into the transmission by the drive input or drive output shafts. Vibration of loose components can also be produced by the self-excitation of transmission-internal components, which can as a rule be attributed to tooth defects, play between the tooth flanks and/or imbalances. The difference between rattling and chattering is as follows: loose component instabilities that occur during traction or thrust operation are called rattling, whereas in contrast, chattering only takes place while idling, i.e. while the transmission is in the neutral position.
The loose wheels of a multi-stage change-speed transmission with two countershafts and a central main shaft are mounted on the main shaft with some radial play in order to enable load equalization between the force flows via the two countershafts, and are guided radially by the teeth of the fixed wheels on the countershafts within the range of their radial movement play on the main shaft, so that when a gear is engaged the corresponding loose wheel is centred under load between the teeth of the fixed wheels on the countershafts. During torque transmission this gives the optimum load equalization between the countershafts. The loose wheels associated with unengaged gears are in a load-free condition and float in the radial direction and in the circumferential direction within the tooth play range.
In the axial direction the loose wheels are as a rule held in place on the main shaft by thrust washers, with each loose wheel between two thrust washers arranged on the main shaft. Yet, there is also some play in the axial direction, which results from manufacturing tolerances and a gap required for oil lubrication.
However, the radially movable mounting of the loose wheels and the axial play result in an unpleasant rattling noise during operation, because the gearwheels of the gears that co-rotate while not under load, which are not involved in the force flow, undergo accelerations in the circumferential, axial and radial directions caused by manufacturing imprecision and rotation oscillations from the drive input and/or output. These accelerations move the loose wheels within the range of their play clearance in such manner that they impact against adjacent components. In particular, the tooth flanks of the engaged teeth impact against one another, this being the main cause of the rattling noise produced by co-rotating gears which are not under load.
From DE 103 34 460 A1 by the present applicant a drivetrain of a motor vehicle is known, in particular a vehicle with all-wheel drive, which comprises a change-speed transmission driven by a motor, in which at least one brake is engaged in the drivetrain, by means of which rotational oscillations in the drivetrain usually caused by shifting under load can be braked.
In this case the at least one brake is preferably arranged in the transfer case; alternatively, it can be arranged between the transfer case and the rear wheels, or before or after the change-speed transmission in the drive input direction.
In DE 103 34 460 A1 it is also proposed to arrange sensors in the drivetrain in order to detect changes under load; in this way, after changes under load have been detected by the sensors the drivetrain can be braked. Here, the sensors are preferably arranged on the wheels of the vehicle.
From DE 102 34 579 A1 a transmission is known, particularly for a utility vehicle, which contains a transmission brake, a lubrication oil pump and at least one countershaft, such that the transmission brake and the lubrication oil pump form a unit connected to a countershaft.
From DE 10 2006 008 207 A1 a method is known for reducing undesired noise, in particular rattling noises, in a parallel-shift transmission, in particular an inactive partial transmission of a parallel-shift transmission, such that the parallel-shift transmission comprises at least two partial transmissions, with a plurality of gears, that act on a common drive output shaft, and a respective clutch for each partial transmission, such that a driveshaft of a motor can be brought into torque engagement with the output shaft selectively by means of one of the partial transmissions with its associated clutch engaged. In this case it is proposed that if undesired noise occurs, the engaged clutch of the active part-transmission should be actuated so as to modify its slip. In particular, it is proposed to actuate the engaged clutch of the active part-transmission in such manner that its slip increases.
Furthermore, from DE 10 2004 057 126 A1 a device is known for reducing the axial movement of loose wheels arranged on the main shaft in a transmission with at least two countershafts, such that for at least one of the wheels on the main shaft a pressing device arranged on the main shaft presses the associated main shaft wheel in the axial direction against a thrust washer. This is intended to avoid undesired rattling noises.
In addition, from the not yet published document DE 10 2008 000 820 by the present applicant a device for reducing rattling noises in a multi-stage transmission with two countershafts is known, in which at least one gearwheel in at least one gear of the multi-stage transmission has conical teeth, such that the gearwheel with the conical teeth or a gearwheel that meshes with the gearwheel is mounted so that it can move axially, and such that the gearwheel having conical teeth and the gearwheel that meshes with it are pressed against one another in the axial direction by at least one elastic element.
In the solutions known from the prior art for rattling noise reduction additional components are used, with adverse effect on the production and assembly costs. Furthermore, these components often act disadvantageously as sources of loss that affect the efficiency of the transmission.
Nowadays, demands concerning the suppression of noise emission by motor vehicles, in particular utility vehicles, are becoming stricter. Particularly in the case of transmissions with a countershaft structure comprising two countershafts and a central main shaft, there is a need for improvement in relation to the reduction of rattling noises.